1. Field of the Invention
The present invention relates to a pickup device and a recording medium drive unit.
2. Description of the Related Art
Conventionally, there has been known a pickup device as a mechanism for recording on and/or reproducing from an optical recording medium. There are various arrangements as the pickup device. Recently, among the various types of pickup devices, a development of a pickup device capable of recording on and/or reproducing from an optical recording medium at higher speed has been promoted.
As an example of those pickup devices, there has been known an arrangement where four linear resilient members connect a lens holder supporting an objective lens and a base, the resilient members swingably supporting the lens holder in a focus direction and in a tracking direction to drive the lens holder.
As a conventional technology for driving the lens holder in such pickup device, an arrangement has been known, in which a printed coil substrate is attached to a lateral side of the lens holder and a magnet is provided at a position away from the printed coil substrate by a predetermined distance (see, for instance, Japanese Laid-Open Patent Publication 2003-45720).
FIGS. 1 to 3 schematically show conventional arrangements.
In FIGS. 1, 2A and 2B, a lens holder 101 supports an objective lens 102, which is adapted to be movable in a tracking direction T in a plane orthogonal to an optical axis of the objective lens 102. A printed coil substrate 103 is attached on a lateral side of the lens holder 101, and a magnet 104 is provided at a position away from the printed coil substrate 103. In the magnet 104, a south pole and a north pole are adjacent to each other.
In the arrangement, when a current is applied to the printed coil substrate 103, currents in inversed directions are applied to both lateral portions 103A, 103B of the printed coil substrate 103, where a driving force Fa is generated between the lateral portion 103A and the opposing south pole and a driving force Fb is generated between the lateral portion 103B and the opposing north pole.
In the magnet 104, since a magnetic field line Mo generated from the north pole toward the south pole is curved, the driving force Fa heads to the lower right in FIG. 1, and the driving force Fb heads to the upper right in FIG. 1.
In the driving forces Fa, Fb, components of forces FaT, FbT along the tracking direction T act effectively as driving forces, the center of the driving forces in the tracking direction is in the vicinity of the lateral sides of the lens holder 101. On the other hand, the center of gravity O of the lens holder 101 substantially coincides with the optical axis of the objective lens 102, so that the center of driving force and the center of gravity O are away from each other in a plane orthogonal to the optical axis of the objective lens 102.
Thus, when the lens holder 101 is moved in the tracking direction T, a torque NT acts around an axis passing through the center of gravity O of the lens holder 101 and parallel to the optical axis of the objective lens 102 in a counterclockwise direction in FIG. 1 due to the components of forces FaT, FbT of the driving forces Fa, Fb. Further, a torque NU also acts in the counterclockwise direction as the torque NT due to components of forces FaU, FbU orthogonal to the components of forces FaT, FbT of the driving forces Fa, Fb.
Since the torques NT, NU act on the lens holder 101, an unwanted rotational resonance is generated in the lens holder 101.
To prevent the inconvenience described above, in the conventional technology, the printed coil substrates 103 are attached to both lateral sides opposing each other of the lens holder 101 in such a manner the printed coil substrates 103 are disposed symmetrically with respect to the center of gravity of the lens holder 101 as shown in FIG. 3.
With such arrangement, since the printed coil substrates 103 are disposed on symmetrical positions with respect to the center of gravity O of the lens holder 101, torques No (torque NT+torque NU) are mutually counteracted, and thereby the center of driving force and the center of gravity of the lens holder 101 coincide with each other.
In the conventional pickup device, since the printed coil substrates 103 disposed on opposed positions about the center of gravity O of the lens holder 101 prevent the unwanted rotational resonance generated in the lens holder 101, reduction in thickness, size, and weight of the device has been limited.
When the printed coil substrate 103 is disposed only on one side of the lens holder 101 to achieve reduction in thickness or the like of the device, a rotational force is generated in the lens holder 101 as described above, so that movement of the lens holder 101 cannot be controlled with high accuracy.